The invention relates to a semiconductor transcalent device and, more particularly, to a transcalent device of a center gate type in which a control or gating means is in communication with the center of one surface of a semiconductor wafer.
The term transcalent device is used to describe a family of high power solid state devices that include integral cooling systems. These devices are constructed to minimize the thermal resistance between a semiconductor wafer, such as silicon or germanium which is the active part of the device, and the utlimate heat sink. Such semiconductor devices include, for example, thyristors, silicon-controlled rectifiers (SCR) and transistors. All these devices produce relatively large amounts of heat, which must be effectively dissipated to prevent breakdown or destruction of the device. Different types of heat sinks have been used. One type utilizes a heat pipe structure affixed to the semiconductor wafer.
One type of heat pipe comprises a porous wick structure in direct contact with the cathode and anode electrodes of the semiconductor wafer. Such a heat pipe is described in U.S. Pat. No. 3,739,235 to S. W. Kessler Jr., issued June 12, 1973, entitled, "Transcalent Semiconductor Device." A second type of heat pipe comprises metal plates of tungsten or molybdeum bonded to the respective cathode and anode electrodes of the semiconductor wafer. The metal plates in turn are conductively bonded to the heat pipe walls. Such a heat pipe structure is described in U.S. Pat. No. 3,984,861 to S. W. Kessler Jr., issued Oct. 5, 1976, entitled, "Transcalent Semiconductor Device." In each of the above-mentioned patents the control electrode for initiated conduction in the semiconductor wafer is disposed at or near the periphery of the wafer adjacent to the cathode electrode. Devices of the above-described type are known as "ring gate" type devices and form a major class of semiconductor devices.
The second major class of semiconductor devices is known as "center gate" devices. In a center gate device the control or gate lead usually contacts a gate electrode located at the center of the silicon wafer. In certain applications a center gate device may be preferably to a ring gate device. For example, in a center gate structure with or without an amplifying gate a smaller area of silicon is necessary for the gate contact than with a ring gate type device. Furthermore, with a center gate it is easier to obtain a higher transient rate of voltage rise (dv/dt) because the periphery of the gate to emitter junction is smaller and therefore spurious electrical leakage and capacitive displacement currents are less than if the periphery were as large as with a ring gate. Also, the emitter area per unit area of silicon wafer, is greater for a center gate device than for a similar size ring gate device resulting in increased current ratings for the same area of silicon.
Semiconductor devices of the "center gate" type are well known in the art. Such devices and various structures for contacting the centrally disposed gate electrode are shown in U.S. Pat. No. 3,296,506, to Steinmetz Jr. et al., issued on Jan. 3, 1967, entitled, "Housed Semiconductor Device Structure with Spring Biased Control Lead," U.S. Pat. No. 3,450,962 to Ferree et al., issued June 17, 1969, entitled, "Pressure Electrical Contact Assembly For a Semiconductor Device," and U.S. Pat. No. 3,599,507 to Lootens, issued on Aug. 10, 1971, entitled, "Semiconductor Device with a Resilient Lead Construction." These above-mentioned devices achieve cooling of the semiconductor wafer by heat conduction through a metal support member. Devices having a metal support member have limited applicability in applications requiring higher current and power ratings where excessive heat is generated, and more efficient means for transferring heat from the semiconductor device is required.
U.S. Pat. No. 3,771,027 to Marek, issued Nov. 6, 1973, entitled, "Bistable Semiconductor Device," describes a center gate type semiconductor device cooled by the heat pipe technique. The Marek structure apparently assumes that the coolant is a nearly perfect dielectric since in FIG. 4 of the patent the coolant is simultaneously in contact with both the emitter and the gate regions of the semiconductor. The shorting problem that may occur in the Marek structure because of the less than perfect dielectric property of the coolant is eliminated by applicants' novel control structure which furnishes both a substantially continuous heat transfer surface in contact with the surface of the wafer and a structure in which the coolant is not in contact with the surface of the wafer.